SQL上的WHERE子句JOIN

Question

我有以下查詢來返回客戶使用的作業（訪問次數）和他/她使用的獨特的獸醫數量。

SELECT 
customer.users_id AS CustID, 
COUNT(VisitID) AS Jobs, 
COUNT(DISTINCT VetID) AS Resources 
FROM customer 
LEFT JOIN visit ON customer.users_id = visit.CustID 
GROUP BY customer.users_id 

當我添加一個WHERE子句來獲得只計算那些沒有改期或取消訪問，我的查詢看起來是這樣的。

SELECT 
customer.users_id AS CustID, 
COUNT(VisitID) AS Jobs, 
COUNT(DISTINCT VetID) AS Resources 
FROM customer LEFT JOIN visit ON customer.users_id = visit.CustID 
WHERE visit.Status != 'Cancelled' AND visit.Status != 'ReScheduled' 
GROUP BY customer.users_id 

然而，在這種情況下，我只得到，雖然我使用的是左連接，爲客戶誰沒有任何取消或重新安排訪問，埃本的客戶。

我該如何適應WHERE子句？

Answer 1

你不想要WHERE條款。這只是將LEFT JOIN變成INNER JOIN。您在LEFT JOIN需要一個AND：

SELECT customer.users_id AS CustID, COUNT(VisitID) AS Jobs, COUNT(DISTINCT  VetID) AS Resources 
FROM customer 
LEFT JOIN visit ON customer.users_id = visit.CustID 
AND visit.Status NOT IN('Cancelled','ReScheduled') 
GROUP BY customer.users_id 

Answer 2

當我們添加一個where子句與left outer join，它像一個inner join，其中ON子句後應用的過濾器。與AND條款取代where

SELECT customer.users_id AS CustID, COUNT(VisitID) AS Jobs, COUNT(DISTINCT VetID) AS Resources 
FROM customer 
LEFT outer JOIN visit 
ON (customer.users_id = visit.CustID) 
AND visit.Status != 'Cancelled' 
AND visit.Status != 'ReScheduled' 
GROUP BY customer.users_id